1. Field of the Invention
The present invention relates to a liquid crystal composition suitable for a field effect liquid crystal display device driven by multiplex driving and, more particularly, to a liquid crystal composition suitable for use in a television image display device.
2. Description of the Prior Art
A liquid crystal matrix display using a field effect twisted nematic (TN) mode has been used in a variety of image display devices such as a display device of a computer terminal and a television receiver. Matrix type liquid crystal devices are frequently used in such image display device. The matrix type liquid crystal devices include a plurality of pixels arranged in a matrix form at a plurality of intersections of a plurality of electrodes formed on the inner surfaces of opposed substrates. These pixels are multiplex-driven.
In recent years, a strong demand has arisen for a large screen and high image quality in the image display device. For this purpose, the number of pixels must be increased, and contrast must be improved.
A negative display type TN liquid crystal device is used as a liquid crystal device capable of displaying a television image since this liquid crystal device visually has high contrast and can perform clear gradation display. In order to increase the number of pixels to increase a resolution or enlarge a display area, the number of scanning lines is inevitably increased, and high-duty multiplex driving must be performed. In such high-duty multiplex driving, however, a difference between the effective voltages of ON and OFF electric fields for turning on and off each pixel is inevitably decreased. Therefore, the operation margin of the drive voltage is reduced and contrast is degraded.
The operation margin and contrast of the liquid crystal display device depend on voltage-luminance characteristics. More specifically, when a change in transmissivity in response to a change in intensity of the electric field applied to the liquid crystal is abrupt, the operation margin can correspondingly be increased and contrast can be improved. Steepness of the voltage-luminance characteristic curve is represented by a ratio .gamma. of voltage V.sub.50 at which transmittance becomes 50% to threshold voltage Vth (referred to as a .gamma. value hereinafter). When the .gamma. value comes close to 1, the change in transmittance is abrupt and the operation margin can be increased. In this case, contrast can also be improved. The liquid crystal display device driven by high-duty multiplex driving must respond quickly since one selection period within which scanning signals are supplied to one scanning line is inevitably shortened.
The .gamma. characteristics have been studied by M. Schadt et al. According to their studies, the .gamma. value representing steepness of the voltage-luminance characteristic curve is represented by equation (1) below and coincides with the measured value: ##EQU1## where V50: the voltage at which transmittance becomes 50%
Vth: the threshold voltage PA1 K11: the splay elastic constant of the liquid crystal PA1 K33: the bent elastic constant of the liquid crystal PA1 .DELTA..epsilon.: the dielectric anisotropy of the liquid crystal PA1 .epsilon..perp.: the dielectric constant in a direction perpendicular to the axis of the liquid crystal molecule PA1 .DELTA.n: the optical anisotropy of the liquid crystal PA1 d: the layer thickness of the liquid crystal PA1 .epsilon.0: the dielectric constant in vacuum PA1 K22: twist elastic constant
According to equation (1), in order to obtain the .gamma. value near 1, the values of the first, second, and third terms must be near 1.
ON and OFF times tON and tOFF of the liquid crystal display device are represented by equations (2) and (3) and coincide with the measured values: EQU tON =.eta.d.sup.2 /{.pi..sup.2 K(V.sup.2 /Vth.sup.2 -1) (2) EQU tOFF=.eta./Kq.sup.2 ( 3) EQU for q=.pi./d, K=K11+(K33-2K22)/4, and ##EQU2## where .eta.: the viscosity
According to equations (2) and (3), the response time of the liquid crystal display device is proportional to the viscosity of the liquid crystal and is proportional to the square of the liquid crystal layer thickness (i.e., the gap between the electrodes) d.
In a conventional liquid crystal display device, in order to improve the .gamma. characteristic, the value of product .DELTA.n.d of optical anisotropy .DELTA.n and liquid crystal layer thickness d is set to fall within the range of 1.0 to 1.1 since the center of the waveform band of visible band falls within the range of 500 to 550 nm. In order to multiplex drive such a conventional liquid crystal display device, transmitted light during the OFF state of the device appears to be colored due to the influence of wavelength dependency which is caused by optical rotatory dispersion during propagation through the liquid crystal layer. In the negative type liquid crystal device, the long wavelength component of the transmitted light has a higher transmittance than the short wavelength component thereof. As a result, the entire screen of the liquid crystal display device undesirably becomes reddish, i.e., sepia.
This is because a bias voltage is always applied to the OFF pixels by drive signals of high-duty multiplex driving. The liquid crystal molecules are slightly inclined from a direction parallel to the substrate surface. As a result, optical anisotropy .DELTA.n of the liquid crystal layer of the OFF pixels is apparently smaller than the value for the initial orientation state, and the transmittance of the long wavelength component is increased.
In order to solve the problem of coloring, the value of product .DELTA.n.d must be increased. In order to increase product .DELTA.n.d, optical anisotropy .DELTA.n and/or thickness d of the liquid crystal layer must be increased. When thickness d is increased, the response time is greatly prolonged because the response time is proportional to the square of thickness d, as described above. A liquid crystal compound having large optical anisotropy .DELTA.n has a higher viscosity and increases the viscosity of the liquid crystal composition, thereby making the response time prolonged. Therefore, it has been required that a liquid crystal composition used in a liquid crystal display device multiplex driven at high speed have a large .DELTA.n and a low viscosity.
In order to improve the .gamma. characteristic, it is seen from equation (1) that ratio .DELTA..epsilon./.epsilon..sub..perp. of dielectric anisotropy .DELTA..epsilon. of the liquid crystal composition to dielectric constant .epsilon..sub..perp. in a direction perpendicular to the axis of the liquid crystal molecule must be decreased. In order to decrease this ratio, the value of dielectric anisotropy .DELTA..epsilon. is decreased, or the value of .epsilon..sub..perp. is increased. If the content of a liquid crystal compound having positive dielectric anisotropy is decreased and the value of .DELTA..epsilon. of the liquid crystal composition is decreased, the response time is prolonged since the value of .DELTA..epsilon. is small. If the content of a liquid crystal compound having a large value of .epsilon..sub..perp. is increased and the value of .epsilon..sub..perp. is large, the viscosity of the resultant liquid crystal composition is increased and the response time is prolonged since the liquid crystal compound having a larger value of .epsilon..sub..perp. has a high viscosity.
A demand has arisen for a liquid crystal composition having a low viscosity, a small ratio .DELTA..epsilon./.epsilon..sub..perp. while the response time is short even if the value of dielectric anisotropy .DELTA..epsilon. is small and the .gamma. characteristic is good.
A conventional liquid crystal composition having a large .DELTA.n is prepared as follows. A polycyclic cyanophenylcyclohexane or cyanobiphenyl liquid crystal compound having a relatively large .DELTA.n is used to prepare a liquid crystal material for increasing the value of .DELTA..epsilon.. A liquid crystal compound containing three or more benzene and/or cyclohexane rings is used to prepare a liquid crystal material for elevating the N-I point. A low-viscosity liquid crystal compound is used to prepare a low-viscosity liquid crystal material. These materials are mixed to obtain a liquid crystal composition having a large .DELTA.n.
However, the value of optical anisotropy .DELTA.n of such a liquid crystal composition is a maximum of about 0.10 to 0.15 and is not sufficiently large.
A liquid crystal composition aiming at increasing .DELTA.n by using a tolan liquid crystal compound having a large .DELTA.n is disclosed in Japanese Patent Disclosure Nos. 61-97383 and 61-97384, and European Patent No. 178937. This liquid crystal composition mainly contains a tolan liquid crystal compound having a large .DELTA.n, a liquid crystal compound having an ester bond, and an NP liquid crystal compound having a cyano group at its terminal and a large .DELTA..epsilon..
Since such a conventional liquid crystal composition contains a tolan liquid crystal compound having a large .DELTA.n, the optical anisotropy of the resultant composition is relatively large. In addition, since the compound having an ester bond is contained in the composition, a smectic phase tends not to appear in a low-temperature atmosphere. A phenyl cyclohexanecarboxylate liquid crystal compound having the ester bond has a small .DELTA.n, and therefore the optical anisotropy of the composition is not so high (about 0.175 at most). The liquid crystal compound having the ester bond has a relatively high viscosity among liquid crystal compounds having a low viscosity. For example, the liquid crystal compound having the ester bond has a viscosity of about 20 cp. For this reason, the viscosity of the liquid crystal composition containing such a liquid crystal compound has a high viscosity, e.g., 28 cp or more. Therefore, a liquid crystal display device using this liquid crystal composition cannot solve the problem of coloring described above since the value of .DELTA.n of the liquid crystal composition is not so large. In addition, the response time of the display device is long because the composition has a high viscosity.
When the content of the tolan liquid crystal compound having a large .DELTA.n is increased in order to increase .DELTA.n in the conventional liquid crystal composition, crystals tend to precipitate since the tolan liquid crystal compound has poor compatibility. Further, when a liquid crystal composition having a large .DELTA.n and a small .DELTA..epsilon./.epsilon..sub..perp. is prepared by reducing the content of the NP liquid crystal compound having large positive dielectric anisotropy .DELTA..epsilon. in order to improve the .gamma. characteristic, the compatibility is degraded and the crystals can easily precipitate. In addition, the viscosity is increased, so that response time is prolonged.
As described above, the conventional liquid crystal compositions do not have a large .DELTA.n and have a high viscosity as well as the narrow temperature range within which the nematic phase appears. As a result, in liquid crystal display devices using the conventional liquid crystal compositions, a display screen is colored, the response time is prolonged, and the usable temperature range is narrow.